1. Field of the Invention
The present invention relates to a light receiving apparatus using a DQPSK (Differential Quadrature Phase-Shift Keying) demodulation method, and a DQPSK demodulation method, and more particularly to a light receiving apparatus using a DQPSK demodulation method and a DQPSK demodulation method, which are adapted for compactly realizing, at a low cost, an apparatus or a system using a DQPSK modulation scheme.
2. Description of the Related Art
The DQPSK modulation/demodulation method which is one of the modulation/demodulation methods in optical communication is a method excellent in utilization efficiency of wavelength, wavelength dispersion tolerance and polarization dispersion tolerance. For this reason, as described in Non-Patent Document 1, it is expected that the DQPSK modulation/demodulation method will be applied to high speed WDM (Wavelength Division Multiplexing) optical communication systems for which these merits are required. The configurations of transmitting/receiving apparatuses of such optical communication systems are shown in FIGS. 1 and 3 of Patent Document 1 and in FIG. 1 of Non-Patent Document 1.
[Patent Document 1] JP-A-2004-516743 (FIG. 1 and FIG. 3)
[Non-Patent Document 1] R. A. Griffin, et al., Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission, Optical Fiber Communication Conference and Exhibit, the United States, March 2002, WX6, pp. 367-368
Here, the configuration and the operation of a typical light receiving apparatus using the DQPSK demodulation method will be described with reference to FIGS. 1 to 4.
FIG. 1 is a block diagram showing a configuration example of a typical light receiving apparatus (demodulating apparatus) 220 using a DQPSK demodulation method.
Light receiving apparatus 220 shown in FIG. 1 comprises beam splitter 228, delay interferometers 230, 232, and balanced photoelectric converters 234, 236.
One of two light signals branched by beam splitter 228 is inputted to delay interferometer 230. Moreover, a light transmission path is formed within one arm 230a of delay interferometer 230 in order to create relative delay time τ with respect to the other arm 230b. 
The other of the two light signals branched by beam splitter 228 is inputted to delay interferometer 232. Moreover, a light transmission path is formed within one arm 232a of delay interferometer 232 in order to create relative delay time τ with respect to the other arm 232b. 
Delay time τ is realized by allowing optical path lengths of the two arms to be different from each other, i.e., by allowing the lengths of arms 230a, 232a to be respectively physically longer than lengths of arms 230b, 232b. 
Further, electrodes are provided at shorter arms 230b, 232b of the delay interferometers 230, 232. Delay interferometers 230, 232 are configured so as to apply a voltage of a reasonable value to each electrode so that phase-shifts of π/4 and −π/4 are fixedly given to a light signal.
FIG. 2 is an explanatory view showing an example of light signal E(t) 214 of typical DQPSK shown in FIG. 1. FIG. 3 is a diagram for explaining the phase state of light signal E(t) 214 of typical DQPSK shown in FIG. 2.
As shown in FIGS. 2 and 3, phase θ of light signal E(t) 214 results in one from among {π/4, 3π3/4, 5π/4, 7π/4} every symbol. An original signal of light signal E(t) 214 is encoded, and is mapped with respect to phase difference Δθ between the original signal and a signal preceding by one symbol. Δθ indicates either one of quaternary, i.e., four-value of {0, π/2, π, 3π/2}. Light signal E(t) 214 includes quaternary, i.e., 2 bits data every symbol. For this reason, in light signal E(t) 214, in the case where the transmission capacity is B [bits/s], symbol speed becomes equal to B/2 [Symbol/s], and the symbol interval becomes equal to τ=2/B sec.
The operation of light receiving apparatus 220 shown in FIG. 1 will now be described.
Received light signal E(t) 214 is branched into two light signals by beam splitter 228. One of the branched two light signals is inputted to delay interferometer 230, and the other light signal is inputted to delay interferometer 232.
Respective delay interferometers 230, 232 serve to further branch inputted light signals into two light signals. Furthermore, each of delay interferometers 230, 232 serves to delay the light signal of one side by τ thereafter to allow it to interfere with the light signal of the other side. Namely, each of delay interferometers 230, 232 allows light signals to interfere with a light signal preceding by one symbol to extract phase difference Δθ.
At this time, delay interferometer 230 allow a light signal of longer arm 230a to interfere with a light signal obtained by shifting the phase of a light signal of shorter arm 230b by π/4. For this reason, light intensities I1, I2 of two light output signals E1(t), E2(t) outputted from delay interferometer 230 are expressed as formula 1.
                                          I            ⁢                                                  ⁢            1                    =                                                                                      E                  ⁢                                                                          ⁢                  1                  ⁢                                      (                    t                    )                                                                              2                        ∝                          1              +                              cos                ⁡                                  (                                      Δθ                    -                                          π                      4                                                        )                                                                    ⁢                                  ⁢                              I            ⁢                                                  ⁢            2                    =                                                                                      E                  ⁢                                                                          ⁢                  2                  ⁢                                      (                    t                    )                                                                              2                        ∝                          1              -                              cos                ⁡                                  (                                      Δθ                    -                                          π                      4                                                        )                                                                                        (                  Formula          ⁢                                          ⁢          1                )            
Balanced photoelectric converter 234 squares and detects two light output signals E1(t), E2(t) to convert the detected light output signals into electric signal x1(t) as shown in formula 2 in accordance with the difference between light intensities I1, I2 of light output signals E1(t), E2(t).
                                          x            ⁢                                                  ⁢            1            ⁢                          (              t              )                                ∝                                    cos              ⁢                                                          ⁢              Δθ                        +                          sin              ⁢                                                          ⁢              Δθ                                      =                  {                                                                                                                1                      ⁢                                                                                          ⁢                      for                      ⁢                                                                                          ⁢                      Δθ                                        =                    0                                    ,                                                                              π                  2                                                                                                                                                                        -                        1                                            ⁢                                                                                          ⁢                      for                      ⁢                                                                                          ⁢                      Δθ                                        =                                                                  3                        ⁢                        π                                            2                                                        ,                                                            π                                                                        (                  Formula          ⁢                                          ⁢          2                )            
Similarly, delay interferometer 232 allows a light signal of longer arm 232a to interfere with a light signal obtained by shifting the phase of a light signal of shorter arm 232b by −π/4. For this reason, light intensities I3, I4 of light output signals E3(t), E4(t) outputted from delay interferometer 232 are expressed as formula 3.
                                          I            ⁢                                                  ⁢            3                    =                                                                                      E                  ⁢                                                                          ⁢                  3                  ⁢                                      (                    t                    )                                                                              2                        ∝                          1              +                              cos                ⁡                                  (                                      Δθ                    +                                          π                      4                                                        )                                                                    ⁢                                  ⁢                              I            ⁢                                                  ⁢            4                    =                                                                                      E                  ⁢                                                                          ⁢                  4                  ⁢                                      (                    t                    )                                                                              2                        ∝                          1              -                              cos                ⁡                                  (                                      Δθ                    +                                          π                      4                                                        )                                                                                        (                  Formula          ⁢                                          ⁢          3                )            
Balanced photoelectric converter 236 squares and detects two light output signals E3(t), E4(t) to convert detected light output signals into electric signal y2(t) as shown in formula 4 in accordance with the difference between light intensities I3, I4 of light output signal E3(t), E4(t). An output example of balanced photoelectric converters 234, 236 is shown in FIG. 4.
                                          y            ⁢                                                  ⁢            2            ⁢                          (              t              )                                ∝                                    cos              ⁢                                                          ⁢              Δθ                        -                          sin              ⁢                                                          ⁢              Δθ                                      =                  {                                                                                                                1                      ⁢                                                                                          ⁢                      for                      ⁢                                                                                          ⁢                      Δθ                                        =                    0                                    ,                                                                                                  3                    ⁢                    π                                    2                                                                                                                                                                        -                        1                                            ⁢                                                                                          ⁢                      for                      ⁢                                                                                          ⁢                      Δθ                                        =                                          π                      2                                                        ,                                                            π                                                                        (                  Formula          ⁢                                          ⁢          4                )            
It is to be noted that, on the light transmitting apparatus side, an original electric signal is encoded so that electric signals x1(t) and y2(t) obtained above at receiving apparatus result in original electric binary signals before encoding at transmitting apparatus, and is mapped to phase difference Δθ of transmitted optical signal E(t) 214.
However, in the light receiving apparatus using the DQPSK demodulation method shown in FIG. 1, there are problems as described below.
The first problem is that the apparatus configuration is not suitable for downsizing. The reason thereof is that two sets of optical modules (optical components) such as a delay interferometer and a balanced photoelectric converter are required for DQPSK demodulation.
The second problem is that the apparatus configuration is not suitable for realization of a low-cost apparatus. The reason thereof is that since two sets of optical modules are required as stated above and that since these optical modules are constructed so that optical components are assembled, the realization of low cost based on mass-production, as in semiconductor integrated circuit production, cannot be expected in the manufacturing process for the light receiving apparatus.